WHERE <<custom function>> IS NULL ... doesn't work

I want to run this query :

SELECT ifnull(a.field1, a.field1) as name FROM table a WHERE ifnull(a.field1, a.field1) IS NULL

But I get an doctrine exception :

Doctrine\ORM\Query\QueryException' with message '[Syntax Error] line 0, col 518: Error: Expected Doctrine\ORM\Query\Lexer::T_IS, got '('' in /library/Doctrine/ORM/Query/QueryException.php:52

If I don't use the IS NULL in my where clause it is working perfectly.

I think this issue is related to :
http://www.doctrine-project.org/jira/browse/DDC-1472